As is conventionally known, projecting an image onto a plane at an angle of elevation or depression causes keystone distortion. This is a common problem for projections where projectors are positioned below the screen to project upwards to the screen. The resulting images are generally known as having a “top-heavy” appearance.
FIGS. 1A, 1B and 2 illustrate the effect of off-axis projection. Specifically, FIG. 2 shows a projection configuration of a projector 5 and a screen 7 for the square image of FIG. 1. The ideal projection shown in FIG. 1A is from the ideal screen shown positioned along the y-axis and the top-heavy distorted projection shown in FIG. 1B is from an actual screen positioned along a v-axis. As can be seen the projected image contains top-heavy distortion of the square image and that the non-uniform spacing grid is due to perspective effect as shown in FIG. 2. The size of the distortion depends on projection angle θ, distance d from the projector 5 to the screen 7, and projector aperture 2α. FIG. 2 shows distortion along one axis only. Turning to FIG. 3, there is shown a projection diagram for the square image of FIG. 1 distorted along two axis by the tilt and pan angles θ, φ associated with projector 5 such that the distorted image shown on the projection screen (i.e. actual screen shown) results.
One type of prior art approach to correcting distortion in projection systems is to use mechanical systems to move the optical axis in order to compensate for actual distortion. However, such mechanical prior art system generally lack flexibility in that compensation is essentially limited to correcting one axis, usually along the vertical axis.
Another prior art solution is disclosed in U.S. Pat. No. 5,465,121 to Blalock et al. which uses a data processing system to correct distortion by removing pixels as necessary using a simple height and pixel equation. This system is, however, only effective for distortion along one axis, namely the vertical axis, and accordingly does not properly compensate for distortion along two or more axis.
Another prior art solution is described in U.S. Pat. No. 6,367,933 to Chen et al., namely a method for correcting keystone distortion resulting from off-axis projection in two angles, horizontal, pan, and vertical, tilt. The method performs a sequence of two one-axis keystone distortions to achieve a two-dimensional correction. First a correction for a tilt angle is corrected and then the resulting image is rotated by 90 degrees. The rotated image is corrected for a vertical angle distortion again but equivalent to the horizontal pan angle. The final image is obtained by rotating back the image by −90 degrees. Rotation by +/−90-degrees imposed intermediate memory to store rotated the whole or parts of rotated image. This approach introduces processing delays and memory cost. Further, this method is very specific to horizontal and vertical angles distortions only since rotation by +/−90 degrees can be achieved without using very complex interpolation techniques. If a roll angle were applied as well such as that shown in FIG. 4 (or any other distortion like radial lens distortion are applied to the image) then this method would require another specialized module to handle the situation, leading to processing delays and increased cost.
It is therefore desirable to provide a general method and system to correct multiple axial displacement distortion in images, which addresses, in part, some of the shortcomings of providing undistorted images as discussed above.